When you use the iPad or iPhone app to play a video while on the same network as a PlayStation 3 (also running Netflix), you'll see an option to watch on your device or the PlayStation. Opt for the latter, and you can control the PS3 playback with your iOS device.

It's a very cool feature that must have been implemented only recently (which is probably why it's poorly documented). Unfortunately, this trick seems to be limited to the PS3. As long as Netflix can see which devices you're logged in on, however, it should be able to control them as well. We can expect to see more of this cross-device app action in the future, especially as cloud apps and services like this become more and more prevalent.